1. Field of the Invention
The present invention relates to a circuit structure of a pixel region of an active matrix type display device using a thin film transistor (TFT), and particularly to a structure of an auxiliary capacitance.
2. Description of the Related Art
In recent years, the demand for an active matrix type liquid crystal display device has been increasing, and a technique for manufacturing a thin film transistor (TFT) using a semiconductor thin film (several tens to hundreds nm in thickness) formed on a substrate having an insulating surface has been rapidly developed.
In the active matrix type liquid crystal display device, a thin film transistor is disposed for each of several tens to millions of pixels arranged in matrix, and an opposite electrode is provided on an opposite substrate for each of pixel electrodes through an intervening liquid crystal so that a kind of capacitor is formed. A voltage applied to each of the pixel electrodes is controlled by the switching function of the TFT and an electric charge to the capacitor is controlled, so that the liquid crystal is driven and the amount of transmitting light is controlled, whereby a picture is displayed.
Since the holding capacitance of the capacitor is gradually decreased by an electric current leak, the amount of transmitting light is changed and deterioration in the contrast of picture display has been caused.
Then, a capacitive wiring line is conventionally provided so that a capacitor (auxiliary capacitance) different from the capacitor with the liquid crystal as a dielectric is disposed in series. This auxiliary capacitance serves to compensate an electric charge lost by the current leak from the capacitor with the liquid crystal as the dielectric.
As typical structures of this auxiliary capacitance, structures using, as a dielectric, (1) a gate insulating film, (2) an interlayer insulating film, and (3) a passivation insulating film are enumerated. Especially, since the structure of (1) has large capacitance per unit, it is often used.
However, conventionally, if an auxiliary capacitance using a capacitive wring line is formed to secure sufficient capacitance, an aperture ratio is lowered. Particularly, in such a small (not larger than 3 inches) and highly fine panel as is used for a projector display device, since a pixel area is small, lowering of the aperture ratio due to the occupied area of the capacitive wiring line has been a problem.
As a phenomenon intrinsic to a display device using a liquid crystal, a light leak occurs from slightly poor orientation of the liquid crystal. Although the liquid crystal is arranged with orientation of some regularity between a pixel electrode and an opposite electrode, there may occur a case that the orientation is disturbed due to roughness of the surface of the electrode, poor rubbing, or the like. The light leak generated in the portion between a region where the orientation of the liquid crystal is disturbed and a region where the orientation of the liquid crystal is normal is called disclination. This disclination lowers display contrast.
As means for preventing this disclination, means for concealing the portion of occurrence of disclination by a black mask (BM) has been adopted. However, since this disclination is generated by a slightly poor orientation of the liquid crystal, it has been difficult to predict the portion of occurrence. Besides, there has been a problem that the aperture ratio is lowered by providing the black mask for concealing the light leak.
The present invention has been made to solve the foregoing problems and an object of the present invention is to provide an active matrix type display device having a sufficient auxiliary capacitance and a high aperture ratio.
According to a first aspect of the present invention, an active matrix type display device is characterized in that the active matrix type display device includes a pixel electrode and a black mask arranged in matrix over a substrate, a thin film transistor is connected to the pixel electrode, a capacitance is connected to the thin film transistor, and the capacitance is made up of the black mask, an inorganic layer being in contact with the black mask, and the pixel electrode being in contact with the inorganic layer.
According to a second aspect of the present invention, an active matrix type display device is characterized in that the active matrix type display device includes a pixel electrode and a black mask arranged in matrix over a substrate, a thin film transistor is connected to the pixel electrode, a first capacitance and a second capacitance are connected to the thin film transistor, the first capacitance is made up of the black mask, a first inorganic layer on the black mask, and the pixel electrode on the inorganic layer, and the second capacitance is made up of the black mask, a second inorganic layer under the black mask, and a drain electrode under the second inorganic layer.
The active matrix type display device of each of the foregoing aspects is characterized in that the black mask is made of a valve metal.
The active matrix type display device of each of the foregoing aspects is characterized in that the respective inorganic layers constituting the respective capacitances include a film selected from the group consisting of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, a DLC (diamond-like carbon) film, an aluminum oxide film, a tantalum oxide film, and a titanium oxide film, or a laminated film of these films.
In addition, the active matrix type display device of each of the foregoing aspects is characterized in that an edge of the black mask is taper-shaped.
Incidentally, the active matrix type display device of each of the foregoing aspects is characterized in that the black mask is formed on an organic resin film.
According to a third aspect of the present invention, a method of manufacturing an active matrix type display device comprises a first step of forming a thin film transistor over a substrate having an insulating surface, a second step of forming a first interlayer insulating film to cover the thin film transistor, a third step of forming a second interlayer insulating film to cover the first interlayer insulating film, a fourth step of forming a black mask to cover the second interlayer insulating film, a fifth step of forming an inorganic layer to cover the black mask, a sixth step of selectively removing the black mask and the inorganic layer, a seventh step of forming a third interlayer insulating film to cover the black mask and the inorganic layer, an eighth step of selectively removing the third interlayer insulating film to expose the inorganic layer, and a ninth step of forming a transparent conductive film to cover the third interlayer insulating film, and the method is characterized in that an auxiliary capacitance is formed of the black mask, a pixel electrode, and the inorganic layer as a dielectric.
The method of the third aspect is characterized in that in the fifth step, the inorganic layer includes a film selected from the group consisting of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, a DLC (diamond-like carbon) film, an aluminum oxide film, a tantalum oxide film, and a titanium oxide film, or a laminated film of these films.
The method of the third aspect is characterized in that in the fifth step, the inorganic layer is formed by anodic oxidation of the black mask made of a valve metal.
According to a fourth aspect of the present invention, a method of manufacturing an active matrix type display device comprises a first step of forming a thin film transistor over a substrate having an insulating surface, a second step of forming a first interlayer insulating film to cover the thin film transistor, a third step of forming a second interlayer insulating film to cover the first interlayer insulating film, a fourth step of forming a black mask made of a valve metal to cover the second interlayer insulating film, a fifth step of selectively removing the black mask, a sixth step of forming an inorganic layer by anodic oxidation of the black mask, and a seventh step of forming a transparent conductive film to cover the inorganic layer, wherein an auxiliary capacitance is formed of the black mask, a pixel electrode, and the inorganic layer as a dielectric.
The method of the fourth aspect is characterized in that in the fifth step, an edge of the black mask is formed into a taper shape.
The method of each of the foregoing aspects is characterized in that the black mask is formed on the second interlayer insulating film made of an organic resin.